CIITA (class II transactivator) is the master transcriptional activator regulator of both classical and nonclassical (DM and DOa) MHC-II genes. Mutations within this gene form the genetic basis for the immunodeficiency, type II group A Bare Lymphocyte Syndrome (BLS). CIITA functions by interacting with transcription factors that directly bind the MHC-II promoter such as RFX/CREB and NF-Y. CIITA is an early step of promoter loading, and is required for subsequent epigenetic changes including the recruitment of histone acetylases and methylases to MHC-II promoters. In addition to MHC-II, we recently identified Plexin A1 (Plxna1) as a novel gene that is also regulated by CIITA. This was accomplished by profiling cDNA isolated from primary dendritic cells (DC) obtained from CIITA-deficient mice versus wildtype mice. Plexins comprise a large gene family and are considered the receptor for semaphorin family members. They were initially identified in neurons as important for neuronal guidance and axonal growth and serve as either retractive or attractive signals to guide axonal extension. We use short-hairpin RNA (shRNA) to block Plxna1 in DCs and show that Plxna1 is important for antigen-specific T cell stimulation. Plxna1 shRNA blocks the ability of DCs to stimulate T cells by >80%. Plxna1 is not involved in peptide processing or antigenic-peptide binding. Preliminary data suggest it functions in Rho but not cdc42 or Rac activation, and is important in actin polarization. Additional data suggest that Semaphorin 6D (Sema6D) is a candidate T cell ligand for Plxna1 on DC. We elect to study three major aspects of the Plxna1 and Sema6D pair in primary DC and T cells: (1) The transcriptional regulation of Plxna1 by CIITA, by other transcription factors and by histone modifying enzymes; (2) the functional effects of Plxna1 on the Rho GTPase pathway; and (3) the interaction of Sema6D on T cells and PlxnA1 on DC, and the T cell signaling pathway that is activated upon Sema6D engagement. PUBLIC HEALTH RELEVANCE: This project focuses on two new molecules, Plexin-A1 and Semaphorin 6D, that control T lymphocyte activation. Plexin-A1 and Semaphorin 6D are important for the optimal activation of T cells by dendritic cells. T lymphocyte activation is central in immunology, and is important in autoimmunity, infectious diseases, vaccine development among other clinically-relevant issues.